1. Field of the Invention
Exemplary aspects of the present invention generally relate to a conductive member, a process cartridge including the conductive member, and an image forming apparatus using the process cartridge.
2. Description of the Background Art
Conventionally, in image forming apparatuses such as a laser printer, a facsimile, or the like using an electrophotographic technique, a conductive member is used for a charging roller serving as a charging member for charging an image bearing member (hereinafter also referred to as a photoreceptor), and a transfer roller serving as a transfer member for transferring a toner image onto the image bearing member to a recording medium.
FIG. 1 illustrates one example of a related art image forming apparatus 300 using the electrophotographic technique. The related art image forming apparatus includes at least an image bearing member 211 on which an electrostatic latent image is formed; a charging roller 212 for charging the image bearing member 211 by abutting the image bearing member 211; a laser beam 213 serving as an exposure mechanism; a developing unit 220 including a toner bearing member (a developing roller) 214 for adhering toner 215 to the electrostatic latent image on the image bearing member 211; a transfer member (transfer roller) 216 for transferring the toner image on the image bearing member 211 to a recording medium 217; and a cleaning unit 221 including a cleaning member (a cleaning blade), 218 for cleaning the surface of the image bearing member 211 after transfer processing. In FIG. 1, reference numeral 219 denotes waste toner.
As illustrated in FIG. 1, in the related art image forming apparatus 300, the charging roller 212 charges an image bearing member 211 while abutting the photoreceptor 211. When a direct current (DC) voltage is applied to the charging roller 212 in contact with the image bearing member 211 from a power source, not shown, the surface of the image bearing member 211 is uniformly charged. Immediately after that, when the surface of the image bearing member 211 is irradiated with the laser beam 213 in accordance with image data, an electrical potential (hereinafter “potential”) of the irradiated portion of the image bearing member 211 is reduced. In such a charging mechanism, in which the surface of the image bearing member 211 is charged by the charging roller 212, it is known that there is discharge across a tiny gap between the charging roller 212 and the image bearing member according to Paschen's law.
When the surface of the image bearing member 211 is irradiated with the laser beam, a potential distribution according to the image is formed thereon, that is, the electrostatic latent image is formed on the image bearing member 211. When the portion of the image bearing member 211 on which the electrostatic latent image is formed passes the developing roller 214, the toner 215 adheres to the electrostatic latent image in accordance with the potential, thereby forming a visible image, that is, a toner image.
The recording medium 217 is transported to the portion of the image bearing member 211 on which the toner image is formed, and the toner image is transferred onto the recording medium 217 by the transfer roller 216. Subsequently, the recording medium 217 is separated from the image bearing member 211. The recording medium 217 is transported through a conveyance path, thermally fixed by a fixing unit (not shown), and discharged outside the image forming apparatus.
After such transfer processing is completed, the surface of the image bearing member 211 is cleaned by the cleaning blade 218 of the cleaning unit 221. Furthermore, a quenching lamp, not shown, removes residual charge so as to prepare the image bearing member for the subsequent image forming processing.
Japanese Patent Laid-Open Application Publication No. Sho 63-149668 and Japanese Patent Laid-Open Application Publication No. Hei 01-267667 disclose a contact-type charging method using the known charging roller described above. In the contact-type charging method, the charging roller is configured to charge the image bearing member by contacting the image bearing member. However, such a contact-type charging method has the following drawbacks.
A substance constituting the charging roller may seep out from the charging roller and transfer to the surface of the device to charge, for example, the image bearing member, leaving marks, or so-called “traces of charge roller”, on the surface of the device to charge.
Furthermore, when an alternating current (AC) voltage is applied to the charging roller, the charging roller in contact with the image bearing member may vibrate. Consequently, there is a possibility that noise is generated.
Moreover, toner on the image bearing member may adhere to the charging roller. In particular, due to the substance seeping out from the charging roller, the toner is more likely to stick to the charging roller. Thus, the charging ability of the charging roller may deteriorate.
Yet further, when the material constituting the charging roller sticks to the image bearing member, and the image bearing member is not in operation for an extended period of time, permanent deformation of the charging roller may occur.
In an attempt to solve problems of this kind, Japanese Patent Laid-Open Application Publication No. Hei 03-240076 and Japanese Patent Laid-Open Application Publication No. Hei 04-358175 disclose a non-contact type charger. In such a non-contact type charger, a charging roller is disposed across from the image bearing member such that a gap, or the closest distance between the charging roller and the image bearing member, is configured to be in a range of from 50 μm to 300 μm, for example. When the charging roller is supplied with voltage, the charging roller can charge the image bearing member.
In such a non-contact type charger, the charging roller and the image bearing member are not in contact with each other, thereby preventing such problems as adherence of the substance composing the charging roller to the image bearing member surface and permanent deformation of the image bearing member described above.
Furthermore, in the non-contact type charger, a smaller amount of toner sticks to the charging roller to begin with, and therefore a smaller amount of toner and the like on the image bearing member sticks to the charging roller.
The non-contact type chargers disclosed in Japanese Patent Laid-Open Application Publication No. Hei 03-240076 and Japanese Patent Laid-Open Application Publication No. Hei 04-358175 are provided with a spacer ring attached at both ends of the charging roller so that a predetermined gap is secured between the charging roller and the image bearing member.
However, according to non-contact type chargers of this type, precise control of the size of the gap is difficult to achieve. Thus, there is a problem such that when the dimensional accuracy of the charging roller and the spacer rings varies, the size of the gap between the charging roller and the image bearing member may fluctuate. As a result, the charge potential of the image bearing member may fluctuate, which is undesirable. Therefore, the main challenge facing such non-contact type chargers is how to maintain a constant gap between the charging roller and the image bearing member so as to ensure a consistent charge to the image bearing member.
In an attempt to solve the above-described problem, Japanese Patent Laid-Open Application Publication No. 2002-139893 discloses a tape-type gap retainer designed to maintain a constant gap between the charging roller and the image bearing member even as the ambient temperature and humidity fluctuates. However, when the charger having the tape-type gap retainer is in use for an extended period of time, there may be a problem such that the tape-type gap retainer is worn out. Furthermore, toner may advance into a space between the charging roller and the tape-type gap retainer, and firmly stick therebetween due to an adhesive agent seeping out from the tape-type gap retainer. As a result, a constant gap between the surface of the image bearing member and the charging roller may not be consistently maintained.
In yet another attempt to solve the above-described problem, Japanese Patent Laid-Open Application Publication 2004-354477 discloses a charging member (a charging roller) including a gap retainer provided at both ends of an electrical resistance adjusting layer.
Referring now to FIG. 2, there is provided a cross-sectional view illustrating the related art charging member (a charging roller). As illustrated in FIG. 2, a charging member (charging roller) 310 includes a conductive supporting member 301, an electrical resistance adjusting layer 302 formed on the conductive member 301, and a spacer 305 serving as a gap retainer and provided at both ends of the electrical resistance adjusting layer 302.
The spacers 305 are formed of thermoplastic resin having a durometer hardness in the range of from HDD 30 to HDD 70, and a mass loss of no more than 10 mg/1000 cycles using Taber Abraser.
Each spacer 305 of the charger 310 of this type is press-fitted onto both end portions of the electrical resistance adjusting layer 302. Accordingly, the spacer 305 is formed at both ends of the electrical resistance adjusting layer 302 and abuts the conductive supporting member 301. Moreover, recently, the electrical resistance adjusting layer 302 and the spacers 305 are processed substantially simultaneously, that is, are cut and ground substantially simultaneously in a single continuous process, and therefore it is possible for the spacer of this type to enhance reliability and accurately control the size of the gap.
In the charging member 310, the spacers 305 (the gap retainers) and the electrical resistance adjusting layer 302 are formed of different material in consideration of toner adhesion characteristics. An ion-conductive agent is used as an electrical resistance adjusting agent of the electrical resistance adjusting layer 302, and thus the water absorption of the electrical resistance adjusting layer 302 is high. Consequently, under high-temperature and high-humidity conditions, the electrical resistance adjusting layer 302 may absorb moisture, causing the dimensions of the electrical resistance adjusting layer to fluctuate.
Since the spacers 305 of the charging member 310 are formed of material including an olefin-based resin, insulating characteristics of the spacers 305 and resistance against toner adherence are enhanced. However, an amount of dimensional fluctuation of the spacers 305 under high-temperature and high-humidity conditions is less than that of the electrical resistance adjusting layer 302. As a result, there may be a problem such that the size of the gap G (illustrated in FIG. 12) formed with such high precision between the charger 310 and the image bearing member may fluctuate when ambient conditions change.
As illustrated in FIG. 3, in an attempt to solve the above-described problems, Japanese Patent Laid-Open Application Publication 2006-78967 discloses a conductive member 410 including a conductive supporting member 401, an electrical resistance adjusting layer 402 formed on the conductive supporting member 401, and a gap retainer 405 provided at both ends of the electrical resistance adjusting layer 402.
The conductive member 410 includes a continuous or a discontinuous fixing groove 401a formed on an outer surface of the conductive supporting member 401 in a peripheral direction facing the electrical resistance adjusting layer 402 and/or the gap retainer 405, and a continuous or discontinuous protrusion 402b formed on an inner surface of the electrical resistance adjusting layer 402 and/or the gap retainer 405 in the peripheral direction such that the protrusion 402b is fitted into the fixing groove 401a. 
When the protrusion 402b is provided on the inner surface of the gap retainer 405 in the peripheral direction, the protrusion 402b can be fitted into the fixing groove 401a, thereby preventing the gap retainer 405 from shifting toward the shaft direction due to changes in the dimension of the electrical resistance adjusting layer 405. Accordingly, the gap fluctuation due to changes in ambient conditions can be reduced.
However, an amount of contraction caused by residual stress at a place of the gap retainer 405 where the protrusion 402b is provided differs from a place of the gap retainer 405 where no protrusion is provided. Consequently, there may be a problem such that the shape of the surface of the gap retainer 405 contacting the image bearing member may be uneven, and the changes in ambient conditions may cause the gap size to fluctuate.
Furthermore, it may be difficult to appropriately position the protrusion 402b provided to the gap retainer 405 so as to fit into the fixing groove 401a, and also confirm the fitting position of the protrusion 402b in the fixing groove 401a. Consequently, some experience and skill may be required to position the protrusion 402b at an appropriate position so that the protrusion 402b is fitted into the fixing groove 401a correctly.
FIG. 4 is a cross-sectional view of another related-art charging member. As illustrated in FIG. 4, Japanese Patent Laid-Open Application Publication 2006-330483 discloses a conductive member 510 including a long-length conductive supporting member 501, an electrical resistance adjusting layer 502 formed on the conductive supporting member 501, and a cap-like gap retainer 505 provided at both ends of the electrical resistance adjusting layer 502.
The electrical resistance adjusting layer 502 includes a step portion having at least one step provided at both ends of the electrical resistance adjusting layer 502 in the direction of both ends. The gap retainer 505 is fixed at both ends of the electrical resistance adjusting layer 502 such that the gap retainer 505 contacts at least two surfaces constituting the step portion. A difference in height relative to an outer circumferential surface of the electrical resistance adjusting layer 502 is formed in an outer circumferential surface of each gap retainer 505 such that a certain gap G is formed between the outer circumferential surface of the image bearing member and the outer circumferential surface of the electrical resistance adjusting layer 502 (see G in FIG. 9.)
The conductive member of this kind enables the surface of the image bearing member to be charged without generating abnormal discharge by preventing deformation of the gap retainer due to the peeling of the end portions thereof during cutting of the surface of the gap retainer.
However, similar to the related art disclosed in Japanese Patent Laid-Open Application Publication 2004-354477, there may be a problem such that changes in ambient conditions may cause the dimension of the electrical resistance adjusting layer to change so that the gap retainer may shift in the shaft direction, resulting in the fluctuation of the size of the gap between the charging member and the image bearing member.